The invention relates generally to an air temperature and humidity control device, and more particularly, to an air temperature and humidity control device integrating more than one heat pump.
Conventional air conditioning systems generally do not perform humidity control functions in an energy efficient manner. When humidity control is desired, air conditioners based on direct expansion (DX) may be operated to condense moisture in the air through supercooling. The drier, supercooled air is then reheated for comfort before entering into a facility to be air conditioned. Significant energy is consumed during the supercooling and reheating of the air, which renders the process inefficient. Moreover, water condensation on the metallic DX coils may cause corrosion problems, which increases the maintenance cost of the air conditioning systems.
In light of the need for more efficient humidity control, air conditioning systems with solid desiccant wheels integrated in temperature control units have been developed. The solid desiccant wheel is loaded with a solid desiccant and is positioned just upstream of the temperature control unit so that cooled air transversely passes over a section of the rotating desiccant wheel, during which the moisture in the air is absorbed by the desiccant. The remaining section of the desiccant wheel is reheated so that the absorbed moisture can be desorbed to regenerate the desiccant. While capable of achieving low humidity outputs, systems based on desiccant wheels are space-consuming and inefficient, as energy is required to regenerate the desiccant. Moreover, because the desiccant wheel is relatively cumbersome and not easy to install or uninstall, the capacity and operation of the systems based on desiccant wheels are generally not intended to accommodate a wide range of operations.
In addition to desiccant wheels, humidity control may be achieved using a system having a heat pump coupled to a liquid desiccant loop. The liquid desiccant, such as lithium chloride for example, is cooled and heated by the heat pump. The desiccant loop includes two contact towers loaded with packing materials or two membrane-type contactors for example. Several sprinklers are provided at the top end of the tower to distribute the liquid desiccant (cooled or heated by the heat pump) onto the packing materials, while air is blown from the bottom end of the contact tower as the liquid desiccant trickles down the packing material. As a result of the direct contact between the desiccant and air, water may be absorbed from the air into the desiccant or desorbed from the desiccant into the air. Simultaneously, the air may be heated or cooled by the liquid desiccant. Because of its integration with a heat pump, the liquid desiccant system discussed above requires less energy for desorbing water from the liquid desiccant, i.e. the regeneration of the liquid desiccant.
However, as the operation of the system requires direct contact between numerous streams of liquid desiccant and air, entrainment of liquid desiccant droplets into the air stream is inherent to spraying direct contact technologies. Such liquid desiccant entrainment (or liquid desiccant carryover) can cause corrosion of ductwork and human health issues. Moreover, similar to the desiccant wheels, the contact towers of the above-discussed system are relatively cumbersome in construction and not easy to modulate to accommodate a wide range of operations.
To address prevalent issues associated with direct contact systems, other systems without direct contact include a contactor having at least one contact module with a porous sidewall that is permeable to water vapor and impermeable to the liquid desiccant employed. The contactor may include at least one contact module with a porous sidewall having exterior and interior sides, wherein the interior side of the sidewall defines an internal space in which the liquid desiccant flows. The blower generates an air flow along the exterior side of the sidewall in order to provide desirable temperature and humidity.
The contactors in these non-direct contact systems commonly include a hydrophobic porous material with limited heat transfer potential, but better mass transfer potential when compared to conventional refrigerant evaporator and condensing technologies. In addition, the performance, size and cost of such materials for the hydrophobic porous contactors needed in these systems places a practical limit on the amount of sensible heat removal that can be achieved economically from the incoming air. Building codes may require that a large fraction of outdoor (ambient) be processed and delivered to the conditioned space within a given temperature and humidity range. The contactor-based temperature and humidity control devices may not be able to process the large fraction of outdoor or process air to desirable conditions in a cost-effective and energy efficient manner.